High melting point metal ingots have been prepared by heating a quantity of the metal with a vacuum arc double electrode remelting furnace. Such a device is known by the acronym VADER, and is described in some detail in U.S. Pat. No. 4,261,412. Ingots produced by this method have not been totally useful because of the difficulty in maintaining sufficient heat control of the ingot. As a result, the ingot produced is often coarse grained, brittle and porous. Maintaining sufficient heat to the opposing consumable electrodes can be accomplished with adequate electrical power to maintain a controlled melt delivery. However, because of the inert gas environment, heat losses are too drastic to maintain a controlled heat balance at the top of the developing ingot. A shallow liquid metal zone at the top of this ingot is therefore not maintainable and too rapid solidification of the molten droplets occurs.
The metal droplets consist of a mixture of liquid metal with solid particles in suspension. These particles act as nuclei which produce very fine grains or crystals on solidification. When the solidification is too rapid, the resultant ingot is very porous. Sometimes the droplets do not completely fuse with one another, creating voids and cold shots in the ingot.
Efforts to eliminate the defects produced by this method of casting have not met with adequate success. There has not been a satisfactory method to date for melting and casting ingots of refractive metals using a high melting point process that can result in a fine grained microstructure. The VADER furnace has not been available to cast these metals, except when extremely small amounts of the metal are required.
Accordingly, it is an object of this inve tion to provide a method for casting ingots of high melting point metals, that can maintain a shallow ingot melt pool and thereby provide a fine grained solidified ingot.
More particularly, it is an object of this invention to provide a method for casting ingots of high melting point refractory metals selected from the group consisting of tungsten, rhenium, tantalum and molybdenum, and alloys thereof.
It is a specific object of the present invention to provide a method to produce high melting point metal ingots to achieve unexpected strength and density in those ingots.
Finally, it is an object of the present invention to provide a method of making ingots from refractory metals and the like, using the VADER furnaces.
Other objects will appear hereinafter.